The present invention relates to a heating arrangement, and in particular concerns a heating arrangement for heating rollers to be used in a laminating apparatus.
It is known to provide laminating devices having a pair of heated rollers. Typically, plastic pouches are provided for use with such a device, the pouches comprising two square or rectangular regions of transparent or translucent plastic which are overlaid on one another and sealed together at one edge. The surfaces of the sheets that face one another are at least partly covered with a thermally-activated glue, which is not adhesive at room temperature but which becomes adhesive at elevated temperatures.
A sheet of paper or card, which is slightly smaller than either of the transparent sheets, is placed within the pouch so that one edge of the paper or card abuts the sealed edge of the pouch. The transparent sheets are then laid flat against either side of the sheet of paper or card, with opposing edges of each sheet projecting slightly beyond each edge of the sheet of paper or card.
The pouch is then passed between a pair of heated rollers, which raise the temperature of the glue, thus activating the glue and causing the transparent sheets to adhere to the surfaces of the paper or card and, where the transparent sheets meet, to each other. The sheet of paper or card is then laminated within the pouch.
Conventionally, heating of each roller is achieved by providing a relatively heavy aluminum shoe, which has a curved inner surface lying close to a region of the roller. When the laminating device is switched on, the shoe is heated, usually by resistance heating, and heat energy is transferred from the shoe to the roller by radiation and convection.
Whilst heating the rollers by this technique has been found to be effective, a relatively long length of time that is taken for the rollers to reach working temperature. For instance, a typical machine will not be ready to perform laminating operations for around five to seven minutes after being switched on. For domestic, business and industrial users, this lag is undesirable, and there is a general need for laminating machines which are ready for use more quickly after being switched on.
More recently, it has been proposed to heat the rollers using radiated heat from a lamp or other source of radiation, particularly in frequencies around the near infra red (IR) region of the electromagnetic spectrum, and more particularly within the wavelength range from 700 nm to 2 μm. Using such techniques, the outer surface of each roller can be heated to a high temperature very quickly. It is, however, difficult to control the heating that is imparted to rollers using this technique.
Furthermore, it is crucial that overheating of the rollers does not take place. Rollers of a laminating machine generally comprise a stiff metal cylinder, which is covered with a thin coating of silicone. Silicone is relatively soft, and may be compressed to some extent, thus allowing different shapes and thicknesses of objects to be laminated.
If the silicone covering is heated beyond a certain temperature, however, the silicone will be permanently damaged, losing its elasticity and rendering the roller (and hence, generally, the entire machine) useless.